lib/PublicInbox/Msgmap.pm | 2 +-

diff --git a/lib/PublicInbox/Msgmap.pm b/lib/PublicInbox/Msgmap.pm
index a49b61e8b7f3636eb28cba61ca052c5000b5720c..7e0f34a4a172d0ff4f09271e573cfc9739b1404b 100644
--- a/lib/PublicInbox/Msgmap.pm
+++ b/lib/PublicInbox/Msgmap.pm
@@ -179,7 +179,7 @@ # see scripts/xhdr-num2mid for usage
 sub mid_set {
 	my ($self, $num, $mid) = @_;
 	my $sth = $self->{mid_set} ||= do {
-		my $sql = 'INSERT INTO msgmap (num, mid) VALUES (?,?)';
+		my $sql = 'INSERT OR IGNORE INTO msgmap (num, mid) VALUES (?,?)';
 		$self->{dbh}->prepare($sql);
 	};
 	$sth->execute($num, $mid);